In general, two wire, low current loop circuits are used with a variety of devices, for example, flow measurement devices. These loop circuits typically include a loop current regulating circuit which varies the current in the loop, generally from 4 to 20 mA, according to a signal received from the associated device. For example, when the loop circuit is used with a flow measurement device, such as a flow meter in a typical process control loop, the flow meter provides a signal ranging from 4 to 20 mA which represents the measured flow rate. This signal is then provided to a controller, which compares the signal received from the measurement device to a signal which represents a desired flow rate, or "set point." The controller calculates a corrective signal (which also may be a 4 to 20 mA signal) that is output to a control device such as a control valve. The control device exerts an influence on the process in response to the received corrective signal to bring the process to the desired flow rate.
The device connected to the loop circuit may include various electronic components such as a microprocessor or display device. It is desirable to power these electronic components via the loop circuit, rather than powering the electronic components via a separate power circuit, thereby reducing installation and maintenance costs. However, these electronic components generally require high current, low voltage as opposed to the low current, high voltage typically supplied by the loop circuit. Moreover, the electronic components of the associated device often communicate digitally over the loop circuit via Frequency Shift Key ("FSK") signals. Using the 4 to 20 mA circuit for communication further reduces wiring requirements, in turn providing additional cost reductions. Several FSK protocols exist, including the HART protocol or other protocols which modulate the loop current at audio frequencies. Accordingly, efficient low powered power management components are needed to regulate the distribution of energy to the electronic components without introducing noise onto the loop circuit that can interfere with the digital communication or presenting a complex impedence to the transmitting source which may attenuate or distort the signal.
FIG. 1 illustrates a typical prior art loop current regulating circuit 100. The prior art loop current regulating circuit 100 is connected between positive loop voltage +LOOP and negative loop voltage -LOOP. The loop current regulating circuit 100 includes a current control circuit 112 and a current compare circuit 113. The current compare circuit 113 senses an actual current on the loop and compares it to a current demand signal 114 received from an associated device (not shown). The demand signal 114 provided by the associated device is in response to a sensed process variable. For example, for an associated device which measures the flow rate of a fluid such as a flow meter, the demand signal provided to the current compare circuit 113 represents the sensed flow rate. The current compare circuit 113 then signals the current control circuit 112 to increase or decrease the loop current to meet the current demanded by the current demand signal 114. The current control circuit 112 utilizes a linear shunt regulator to vary the current in the loop in accordance with the signal received from the current compare circuit 113 and to form a pre-regulator circuit for controlling start-up functions at initial application of power.
The linear shunt regulator includes a transistor 115, a zener diode 116, and a resistor 117. The transistor 115 operates linearly and becomes more or less conductive based on the signal from the current compare circuit 113. If the loop current needs to be increased, the transistor 115 becomes more conductive. As the transistor 15 becomes more conductive, the voltage at node 118 increases. When the voltage at node 118 reaches approximately 7 volts, the zener diode 116 will turn on. At this point, all current in excess of the demanded loop current will sink to ground through the zener diode 116.
However, the prior art loop current regulating circuit 100 provides inexact current control at best. In the prior art circuit only one active device is utilized and the path to ground is not controlled by the active device. Rather, the path to ground is through the zener diode 116 and therefore, is only indirectly controlled. Thus, precise current control is not possible. Moreover, the current control of the loop current regulating circuit 100 is not smooth, because of the abrupt nature of the linear shunt regulator: the zener diode 116 is either on or off. Another problem created by the prior art circuit is that the current control circuit creates a complex impedance. This complex impedance can distort FSK signals, if the circuit is used with a device that transmits and receives FSK signals over the loop circuit. If the digital communications signals cannot be reliably transmitted and received over the loop circuit, the cost advantage gained through reduced wiring is lost.
Further, it is desirable to power peripheral electronics of an associated device from the loop circuit 100 at node 118. However, the prior art loop current regulating circuit 100 causes several problems when used to provide power to an associated device. First, the power drain is great due to the loss associated with the zener diode 116. Thus, the prior art loop current regulating circuit 100 is very inefficient. Additionally, the maximum power provided to any associated device is limited, because the voltage at node 118 is limited to a maximum of approximately 7 V by the zener diode 116. Therefore, the loop current regulating circuit 10 provides poor power amplification. Moreover, if FSK signals are transmitted and received using the prior art circuit, noise can be introduced onto the loop by the FSK circuits connected to the loop current regulating circuit 100, which regulates power to the associated device.
Finally, devices employed on low current loop circuits are often used in hazardous areas where electrical energy or sparks could cause disastrous ignition of surrounding explosive gasses or particles. Accordingly, the power management components must be designed to meet the standards set forth for an Intrinsically Safe device. Such devices receive their operating voltage through energy limiting barriers, and must be specially constructed to reduce or eliminate electrical discharges capable of causing combustion of the surrounding materials.
Thus, a need exists for an improved power management circuit that overcomes the referenced and other limitations of prior art power management circuits.